Unless you are EXTREMELY lucky and buy EXACTLY the right board now, you'll want a new board in a year or two anyway as your style and skill level changes. So as Alka says, as long as it's a reasonable size and not busted, $100 for board and bindings is pretty good.

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I told my kids, "Someday, you'll have kids of your own." One of them said, "So will you."
-- Rodney Dangerfield.

I know I will buy a new board next year or the year after. I just don't wanna pay 67$ bucks (about 80 with taxes) for rental everytime I go. Especially knowing that this year, i'll do a 3 day trip with lessons every morning... (I'm trying to get as maximum improvement as I can get, going to Stoneham with bunch of friends who do ski).

On the net, there is also a lot of 5150, Sims, Lamar...

Also, what happens if calculators on the net are giving me a ideal lenght of 156, but i go with a board that's 150? How lenght will affect the ride?

Also, and it's something people keep forgetting about, the one place where height really matters compared to board length is that a shorter board will have holes positioned for a narrower stance. Get too short of a board and you may not be able to position your bindings far enough apart to be comfortable. Pretty sure this requires more than a 6cm difference though.

__________________
I told my kids, "Someday, you'll have kids of your own." One of them said, "So will you."
-- Rodney Dangerfield.